A spinodal is a line or surface on the phase diagram which separates metastable states (phases) from labile i.e. absolutely unsustainable phases. The unstable phase spontaneously decomposes into stable phases. This is an irreversible process which is determined by local fluctuations in the concentrations of mixture components. The difference between spinodal decomposition and decomposition of metastable phases is that the latter involves the formation of nucleation centres and the former occurs uniformly throughout the material.

Spinodal decomposition occurs in a variety of materials: alloys, glasses, gels, ceramics, liquid solutions and polymer solutions. One possible example is the formation of an inhomogeneous mixture with a fine-grained structure in the case of sharp cooling of some solid solutions. Spinodal decomposition in the presence of nanopores can be used to synthesise nanotubes and nanorods.